For most applications of superconductors the critical current density is the true figure of merit--the higher the better. In type II superconductors such as the high-T.sub.c materials, magnetic flux can easily enter the body of the material, i.e., the lower critical field H.sub.c1 is very small. It is important to "pin" such magnetic flux because motion of flux lines implies dissipation. Such pinning action occurs naturally at any sort of defect in the material. There are various techniques to improve the critical current density by introducing artificial defects; irradiation of thin films with neutrons as described by Van Dover et al., Nature, 55, 1989, and with protons as described by T. K. Worthington et al., Phys. Rev. Lett. 65, pg. 1164, 1990 have been reported. The creation of more pinning sites is reasonable as long as the critical current density is limited by flux depinning.
Other methods of artificially increasing the defect density have also been tried including ion irradiation and cation substitution.